SIMULTANEOUS MEASUREMENT OF INTRACELLULAR NA+ AND CA2+ DURING K+-FREE PERFUSION IN ISOLATED MYOCYTES

To study the relationship between intracellular Na+ concentration ([Na+](i)) and intracellular Ca2+ concentration ([Ca2+](i)), guinea pig ventricular myocytes were loaded with both the Na+-sensitive probe, sodium-binding benzofuran isophthalate (SBFI), and the Ca2+-sensitive probe, fluo 3. [Nai]i was measured from the ratio image at 510 nm when excited at 340/380 nm. [Ca2+](i), expressed as the percent change of corrected flue 3 fluorescence, was measured at 540 nm when excited at 500 nm. The fluorescent spectra of these probes were sufficiently different to allow for simultaneous measurement. After 30 min perfusion of K+-free solution, [Na+](i) of rod-shaped cells increased from 6.4 +/- 0.5 to 20.6 +/- 2.6 mM, and [Ca2+](i) increased to 256 +/- 36% of the control. [Ca2+](i) was higher in spontaneously contracting cells and shortened cells than in rod-shaped cells at similar levels of [Na+](i). When Ca2+-free solution or Ni2+ (5 mM) was applied, [Ca2+](i) was lower than in cells perfused uith K+-free solution alone. It was suggested that extracellular Ca2+- and the Na+-Ca2+ exchange were involved in the increase in [Ca2+](i). In conclusion, we have developed a new method for the simultaneous measurement of [Na+](i) and [Ca2+](i) in isolated myocytes, which should be useful to study the relation between [Na+](i) and [Ca2+](i).